The present invention relates to rotary drum filters used in the pulp and papermaking industry to form a mat of wood pulp and separate the filtrate from its mat. The invention relates especially to such filters of the type in which there is a discharge assembly, including a discharge valve member, and a corresponding valve housing, which together are located at one end of the drum filter for controlling the application of suction to the filter surface and the drawing of filtrate from the mat and drum. More particularly, the invention relates to a system and method for fixing the clearance or gap formed between the discharge valve member and the valve housing.
A drum filter of the type described above, such as a pipe machine type drum filter, comprises a rotary drum partially submerged in a tank of pulp slurry. Typically, the drum has axially extending filtrate channels spaced about its peripheral surface defined by ribs and covered by a filter screen. The surface filtrate channels communicate with an external source of subatmospheric pressure, or suction, through drainage pipes, an annular valve housing mounted centrally within the discharge end of the drum, a discharge valve member, typically a stationary vacuum breaker valve member, seated within the valve housing and extending therefrom, and a discharge elbow connected to the outlet end of the valve member. The discharge valve member has a valve closure segment which blocks off a section of multiple discharge ports in the valve housing to control the application of subatmospheric pressure to the connected drainage pipes and surface filtrate channels.
As the drum rotates about its axis with suction applied, the screen rotates through the pulp slurry and collects a wet mat or "cake" of fibers from the slurry. Filtrate is drawn from the mat through the screen and filtrate channels into the drainage pipes, and from there through the valve housing and into the discharge valve member and discharge elbow to remove the filtrate from the drum. During rotation of the drum, the stationary valve closure segment cuts off suction to the filtrate channels in a segment of the drum surface, thereby enabling removal of the pulp mat from the screen surface, usually with the aid of a doctor device.
Rotary drum filters of the above-described type having a discharge valve assembly at one end of the drum are shown, for example, in U.S. Pat. No. 3,363,774 to Luthi and U.S. Pat. Nos. 4,383,877, 4,608,170, 4,608,171, and 4,683,059 all to LaValley. The above-cited issued LaValley U.S. patents are assigned to LaValley Industrial Plastics, Inc. the common assignee of this application, and are incorporated herein by reference and made a part hereof.
Typically, the discharge valve member for such a rotary drum filter, such as stationary valve member 22 of LaValley patents U.S. Pat. No. 4,608,170-1, is seated within the valve housing. A conically tapered valve closure segment extends rearwardly from the plug to cover a portion of the valve housing outlet ports to block them from the source of suction. A large central shaft extends rearwardly from the plug and outwardly beyond the discharge end of the drum, connecting with an annular flange and sealing rim at the outlet end of the valve member. Usually, a hollow generally cylindrical drive trunnion or shaft connected to the drum surrounds the shaft portion of the valve member to define a portion of the filtrate discharge passage leading from the drum. The outlet end of the valve member joins an inlet end of the discharge elbow at respective flanges, bolted together. The outlet end of the elbow, in turn, connects to a suction pipe leading to the source of subatmospheric pressure. The drive trunnion is enclosed within a stationary drive housing. The drive housing houses the drum drive means, typically a worm gear drive.
Means can be provided for the user to manually adjust the position of the elbow flange both axially and transversely of the valve member on the drive housing to achieve proper seating of the segment. Such manual adjustment can be manually positioned with both axial and radial adjustment bolts acting between the elbow flange and the drive housing. Proper seating of the valve closure segment is complicated by the fact that the valve member and discharge elbow must be securely bolted together at their flanges when the seating adjustment is made because the elbow flange connects the valve member to the drive housing. However, when the elbow is connected to the valve member, access to the valve housing and segment end of the valve member is extremely limited, making accurate seating difficult and tedious. Seating can only be accomplished through trial and error using a feeler gauge affixed to a long stick inserted through a small inspection port in the discharge valve member.
In U.S. Pat. No. 4,608,170-1, a vacuum breaker valve-discharge elbow assembly for a rotary drum filter of the pipe machine type is mounted onto the drive gear housing for manually adjusting the seating via a separate mounting plate. A valve flange is secured to the mounting plate. The mounting plate is in turn transversely and axially adjusted manually on the gear housing via adjusting bolts to accurately seat the valve within its valve housing in the drum. The discharge elbow is then secured to the mounting plate independently of the valve. The valve can be rotationally manually adjusted on the mounting plate without disturbing the prior axial and transverse adjustment of the plate and valve by providing slotted fastener access holes on the valve flange and close tolerance valve flange guide shoulders on the mounting plate. Cooperative calibration plates on adjacent valve flange and mounting plate surfaces facilitate precise angular manual adjustment of the valve.
Setting and maintaining the properly aligned seating of the discharge valve assembly within the valve housing during use is critical. The gap formed between the discharge valve member and the valve housing must be centered and set at a fixed clearance which is close enough to produce a maximum vacuum level, but spaced apart to prevent damage from occurring during use due to the interaction of the valve member and the valve housing. However, maintaining the gap at a requisite fixed clearance is a particular problem after manually setting gap when for instance, the drum undergoes normal axial expansion and contraction during use. Thus, in certain instances, such as when the discharge valve assembly is located on the non-driven side of the drum, the drum can expand beyond the gap clearance causing severe damage to the discharge valve assembly and resulting in costly production downtime.
Therefore, a need exists for a system and method for readily setting and maintaining during use the gap between the discharge valve member and the valve housing of a rotary drum filter at a fixed clearance.